The hypophosphatemic (Hyp) mouse, a murine homologue of human X-linked hypophosphatemic rickets, is characterized by renal defects in brush border membrane phosphate transport and vitamin D3 metabolism. The present study was undertaken to examine whether elevated renal 25-hydroxyvitamin D3-24-hydroxylase activity in Hyp mice is associated with increased degradation of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] by side chain oxidation. Metabolites of 1,25(OH)2D3 were separated by HPLC on Zorbax SIL and identified by comparison with standards authenticated by mass spectrometry. Production of 1,24,25-trihydroxyvitamin D3, 24-oxo-1,25-dihydroxyvitamin D3, and 24-oxo-1,23,25-trihydroxyvitamin D3 was twofold greater in mitochondria from mutant Hyp/Y mice than from normal +/Y littermates. Enzyme activities, estimated by the sum of the three products synthesized per milligram mitochondrial protein under initial rate conditions, were used to estimate kinetic parameters. The apparent Vmax was significantly greater for mitochondria from Hyp/Y mice than from +/Y mice (0.607 +/- 0.064 vs. 0.290 +/- 0.011 pmol/mg per protein per min, mean +/- SEM, P less than 0.001), whereas the apparent Michaelis-Menten constant (Km) was similar in both genotypes (23 +/- 2 vs. 17 +/- 5 nM). The Km for 1,25(OH)2D3 was approximately 10-fold lower than that for 25-hydroxyvitamin D3 [25(OH)D3], indicating that 1,25(OH)2D3 is perhaps the preferred substrate under physiological conditions. In both genotypes, apparent Vmax for 25(OH)D3 was fourfold greater than that for 1,25(OH)2D3, suggesting that side chain oxidation of 25(OH)D3 may operate at pharmacological concentrations of substrate. The present results demonstrate that Hyp mice exhibit increased renal catabolism of 1,25(OH)2D3 and suggest that elevated degradation of vitamin D3 hormone may contribute significantly to the clinical phenotype in this disorder.
H S Tenenhouse, A Yip, G Jones
Usage data is cumulative from March 2023 through March 2024.
Usage | JCI | PMC |
---|---|---|
Text version | 88 | 0 |
53 | 7 | |
Scanned page | 76 | 0 |
Citation downloads | 7 | 0 |
Totals | 224 | 7 |
Total Views | 231 |
Usage information is collected from two different sources: this site (JCI) and Pubmed Central (PMC). JCI information (compiled daily) shows human readership based on methods we employ to screen out robotic usage. PMC information (aggregated monthly) is also similarly screened of robotic usage.
Various methods are used to distinguish robotic usage. For example, Google automatically scans articles to add to its search index and identifies itself as robotic; other services might not clearly identify themselves as robotic, or they are new or unknown as robotic. Because this activity can be misinterpreted as human readership, data may be re-processed periodically to reflect an improved understanding of robotic activity. Because of these factors, readers should consider usage information illustrative but subject to change.